The present invention relates to an injection device of the type that receives a syringe, extends it, discharges its contents and then retracts it automatically. Devices of this general description are shown in WO 95/35126 and EP-A-0 516 473 and tend to employ a drive spring and some form of release mechanism that releases the syringe from the influence of the drive spring once its contents are supposed to have been discharged, to allow it to be retracted by a return spring.
However, problems have arisen in devices such as these that make it difficult to ensure both complete discharge of the syringe contents and reliable release of the syringe from the drive spring. Because of the stack up of tolerances of the various components of the device, a certain margin of safety must be built into the activation of the release mechanism, to ensure that it is effective. The consequence of underestimating the safety margin is that the release mechanism may fail to operate even once the syringe contents have been discharged, which is unsatisfactory in a device that is supposed to retract automatically, particularly for self-administered drugs. On the other hand, overestimating the safety margin may mean that some of the syringe contents are discharged after the syringe has retracted, which results firstly in a short dose and secondly in what may be termed a “wet” injection. Wet injections are undesirable for the squeamish, particularly in connection with self-administered drugs.
UK patent applications nos. 0210123, 0229384 and 0325596 describe a series of injection devices designed to deal with this problem. Each makes use of a neat trick that delays the release of the syringe for a certain period of time after the release mechanism has been activated, in an attempt to ensure that the syringe has been completely discharged. The devices illustrated in UK patent applications no. 0325596 make use of a fluid-damped delay mechanism that is particularly effective in ensuring complete discharge of the syringe contents, but creates problems of its own. Firstly, the use of a fluid-damped delay mechanism requires the creation of a fluid-tight reservoir. Thus, the manufacturing tolerances of those components that define the fluid reservoir must be fine, or seals must be used to prevent the fluid from leaking out before its job is done. Secondly, it is undesirable for the fluid to leak out of its reservoir even when the device has been actuated, because that could give rise to a simulated wet injection, or to the impression that the syringe contents may have leaked within the device. Neither is conducive to the peace-of-mind of self-administered drug users. Again, fine tolerances or seals are called for, which pushes up the price of manufacture. For injection devices that are designed to be disposable, as many will be, every penny counts.